1. Field of the Invention
The present invention relates to a friction disc for use in false twisting, and a false twisting spindle provided with such discs.
2. Description of the Prior Art
Texturising by false twisting is a technique which is in itself well known and is widely used, and which consists basically in continuously overtwisting a thermoplastics yarn, fixing the yarn when so deformed, in particular by the action of heat and of a subsequent cooling treatment, and finally untwisting the yarn by the amount to which it was twisted.
It has long been known to impart a false twist to a moving yarn by means of discs (see in particular U.S. Pat. No. 1,030,179 in the name of Hilden) and it has also been proposed to use this idea for false twist texturising, for which purpose friction discs are arranged spaced from one another to rotate on axles which are essentially parallel to one another, the discs overlapping each other (see especially French Pat. Nos. 1,202,393 and 1,255,922 in the name of Scragg and 1,261,747 in the name of Zavody).
The spindles most widely used at present for this purpose in general comprise three rotating axles supporting the discs, the axles being so arranged that as seen in plan view they are at the apices of an equilateral triangle. The discs overlap one another so that the yarn to be treated passes through the unit following a zig-zag path between the discs.
In general, each axle supports several superposed spaced apart discs, the discs of one axle being staggered relative to those of adjacent axles so that they overlap one another.
The friction discs generally have a simple shape, with a convex peripheral surface which the moving yarn contacts, and upper and lower surfaces which are generally planar.
Various materials have been used for producing such friction discs. Currently, the materials which give the best results and which are tending to replace the originally used polyurethanes are ceramic. Ceramics are generally insensitive to chemical agents, which is extremely valuable bearing in mind the number of chemicals used in sizings deposited on the yarns, and furthermore have high resistance to wear and high heat resistance which makes it possible to achieve high speeds, as well as very high hardness and freedom from fouling. Furthermore, the properties of ceramic are easily reproducible, which makes it possible to replace friction discs without altering the quality of the yarn, and which ensures that the quality is uniform from one position of the machine to another. Currently, the majority of friction discs are produced from solid ceramics, which has the advantage of very high reproducibility, though use of discs of which only the surfaces are coated with ceramic has been considered.
The three-axle spindles which are currently marketed essentially consist of a horizontal support on which are perpendicularly mounted the axles carrying their friction discs. These axles are connected to one another and are driven synchronously by a belt connected to a drive pulley.
The friction discs each possess a convex peripheral surface against which the yarn makes contact, and the discs partially overlap one another, with the spacing between two discs generally being of the order of 0.5 millimeter.
These devices are generally satisfactory, but it has been found that during continuous operation a slight deposit can form on the non-working surfaces of the discs, this usually arising from sizings of the yarns. Furthermore, the production of these discs requires rather high precision, in view of the fact that it is necessary that the yarn should, as far as possible, be in contact over an arc of the same length with each friction disc.